Typically, a digital imager array includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, e.g. a photogate, photoconductor, or a photodiode. In a CMOS imager a readout circuit is connected to each pixel cell which typically includes a source follower output transistor. The photoconversion device converts photons to electrons which are typically transferred to a floating diffusion region connected to the gate of the source follower output transistor. A charge transfer device (e.g., transistor) can be included for transferring charge from the photoconversion device to the floating diffusion region. In addition, such imager cells typically have a transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. The output of the source follower transistor is gated as an output signal by a row select transistor.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630 to Rhodes, U.S. Pat. No. 6,376,868 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al., U.S. Pat. No. 6,326,652 to Rhodes, U.S. Pat. No. 6,204,524 to Rhodes, and U.S. Pat. No. 6,333,205 to Rhodes. The disclosures of each of the forgoing patents are hereby incorporated by reference in their entirety.
FIG. 1 illustrates a block diagram of an exemplary CMOS imager device 308 having a pixel array 200 with each pixel cell being constructed as described above. Pixel array 200 comprises a plurality of pixels arranged in a predetermined number of columns and rows (not shown). The pixels of each row in array 200 are all turned on at the same time by a row select line, and the pixels of each column are selectively output by respective column select lines. A plurality of row and column lines are provided for the entire array 200. The row lines are selectively activated by a row driver 210 in response to row address decoder 220. The column select lines are selectively activated by a column driver 260 in response to column address decoder 270. Thus, a row and column address is provided for each pixel. The CMOS imager is operated by the timing and control circuit 250, which controls address decoders 220, 270 for selecting the appropriate row and column lines for pixel readout. The control circuit 250 also controls the row and column driver circuitry 210, 260 such that these apply driving voltages to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal (Vrst) and a pixel image signal (Vsig), are read by a sample and hold circuit 261 associated with the column device 260. A differential signal (Vrst−Vsig) is produced by differential amplifier 262 for each pixel which is digitized by analog-to-digital converter 275 (ADC). The analog-to-digital converter 275 supplies the digitized pixel signals to an image processor 280, which forms a digital image.
In a digital CMOS imager, when incident light strikes the surface of a photoconversion device, e.g., a photodiode, electron/hole pairs are generated in the p-n junction of the photodiode. The generated electrons are collected in the n-type region of the photodiode. The photo charge moves from the initial charge accumulation region to the floating diffusion region or it may be transferred to the floating diffusion region via a transfer transistor. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower transistor (described above).
Image lag can be a problem for imagers, whether the imager is a CMOS, CCD or other type of imager. Image lag can occur, for example, in CMOS image sensor pixels using transfer transistors to transfer charge from the photodiode to the floating diffusion region. There is a potential barrier corresponding to the photodiode/transfer gate region. If this potential barrier is too high, a portion of the charge will be unable to move from the photodiode to the floating diffusion region. The greater the potential barrier, the less charge will be transferred to the floating diffusion region. A potential barrier in the photodiode/transfer gate region may cause incomplete charge transfer reducing the charge transfer efficiency (CTE) of the pixel cell. Charge remaining in the photodiode from a prior image can affect a subsequent image, causing image lag, where a ghost image from the initial charge is apparent in a subsequent image.
Fringing fields improve charge transfer from a photoconversion device, e.g. a photodiode, to a charge collection region. Conventional imagers typically utilize low dielectric (K) oxide spacers for transistor gates, which create smaller fringing fields. A larger fringing field in, for example, a transfer gate of a CMOS imager would improve charge transfer from the photodiode to the floating diffusion region. This would thereby reduce image lag because more carriers are transferred. In CCD imagers, larger fringing fields improve charge transfer efficiency (CTE) in addition to improving image lag characteristics.
CCD devices that use overlapping polysilicon 1 and polysilicon 2 electrodes achieve a high fringing field by applying high voltages to the polysilicon electrodes. This is not desirable on CMOS imagers which are advantageously low voltage devices so they will compatible with CMOS logic circuit and devices. Another imager device, the single polysilicon CCD imager does not have overlapping polysilicon electrodes and could also benefit from a method to achieve high fringing fields to achieve improved charge transfer. Thus, there is a desire and need to increase fringing fields and thereby improve charge transfer and reduce image lag in imager devices.